The International Telegraph and Telephone Consultive Committee (CCITT) Recommendation H.221 provides a recommendation for the format of line transmission of non-telephone signals. In particular, H.221 recommends a standard frame structure for the transmission of one or more "B-channels", for audiovisual services, in an overall transmission channel. A B-channel, or "Bearer Channel", is a 64 kbits/sec channel that carries customer information such as voice-calls, circuit-switched data, or packet-switched data. A B-Channel has a constant, and thus predictable, bandwidth.
The H.221 recommendation divides the overall transmission channel into frames, each frame containing eighty 8-bit octets. The bits at a particular bit position of the octets are collectively regarded as a sub-channel of 8 kbit/sec. The lower transmission rates of the sub-channels are suitable for video, data and telematics purposes. The eighth sub-channel (i.e. the bits of the octets at bit position 8; note that H.221 numbers the octet bits from 1 to 8), called the Service Channel, contains eight bits of frame alignment signal ("FAS"), which allows a receiver to synchronize with the 64 kbits/sec serial stream, and eight bits of bit-rate allocation signal ("BAS"), which allows the receiver to determine the capacity and structure of the information transmitted in the channel. Recommendation H.221 is hereby incorporated by reference to provide additional background information regarding the invention disclosed herein.
Communications information may be transmitted over a Local Area Network ("LAN") or a Wide Area Network ("WAN") in the H.221 frame structure format. FIG. 1 shows a system in which a computing node 42a is connected for audio, video, and data exchange over a WAN and with other, perhaps similar, computing nodes (e.g., node 42b). The computing node 42a is connected by an isochronous Ethernet serial physical layer link 47 to an isochronous-capable hub 40. Alternatively, the computing node 42 may be connected to a private branch exchange ("PBX"). The hub/PBX 40 is connected to a WAN, and backbone connections provide a connection from the hub/PBX 40 to other hubs/PBX's.
The inventors have recognized the desirability of transferring information isochronously between subsystems within the computing node 42. Thus, advantage can be taken of the fixed latency and bandwidth that is characteristic of isochronous communication.